Thursday, February 19, 2015

Cuddle up next to a Cuttlefish....or is that a rock?

By Kara Jew

As
much as I would like to tell you how eating cuttlefish will prevent diseases
and bring a lifetime of health benefits I can’t. We’re friends (maybe
acquaintances), so I won’t lie to you. But hey I’m not going to stop you if you
want to eat cuttlefish. It’s your life, and you can do whatever you want. Although
my advice: don’t eat it with asparagus…especially when vanilla paste is an
option. You’ll upset your stomach and whoever is standing behind you. I’m here
to tell you a little more about what the cuttlefish does and how it’s like the
Mystique (nerd alert: I’m an X-Men fan) of the ocean.

First off, one of the
weird things about the cuttlefish is its eye. Under bright conditions,
the
cuttlefish pupil resembles of Charlie Brown’s mouth, or in other words, their
pupils exhibit a W-shape. Cuttlefish aren't the only ones with peculiar
pupils though. Other organisms have a vertical or horizontal pupil
such as a Madagascar Velvet Gecko or Yellow Mongoose, respectively.

The eye of a cuttlefish. Look at that W!

It has been suggested that this peculiar shape is restricted to
species that spend a considerable amount of time camouflaging on a substrate
(or the surface on which an animal or plant lives). A study found that the
pupil persists as a W-shape when exposed to bright lights but morphs into a
full circle under darkness. Possessing an adjustable pupil allows an organism regulate
the amount of light entering the eye thereby avoiding saturation of
photoreceptors. They proposed that the W-shaped pupil aids in balancing out
light in the natural habitat of the cuttlefish.

On the left you have the vertical pupil of a Madagascar Velvet Gecko, and on the right the horizontal pupil of the Yellow Mongoose

But enough of
eyes! Let’s get to the good stuff. More interesting than the weird eyes of the cuttlefish
is their ability to camouflage.

Cuttlefish and the different looks it can take on.

Perhaps
the most prominent characteristics of the cuttlefish are their elastic
pigmented cells called chromatophores. These are neuromuscular organs, so these
cells change with immense flexibility and speed in response to stimuli. In the video below, Backyard Brains, a company dedicated to making neuroscience
accessible to everyone,
used an iPod to look at the dynamic of a squid's chromatophores. They played
the song “Insane in the Membrane” and watched the chromatophores go to town.

Chromatophores dancing to "Insane in the Membrane" by Backyard Brains.

The chromatophores of cephalopods, like squid,
octopi, and cuttlefish, are used for cryptic and social displays. Within each
chromatophore is a large pigment-containing compartment, the cytoelastic
sacculus, bearing numerous pigment granules that give the chromatophore its
color (red, brown, yellow, orange or black). The cytoelastic succulus is surrounded
by radial muscles that are controlled by motor neurons in the brain. Cuttlefish
can alter its color schemes through contraction or relaxation of these
chromatophores. Specifically, contracting the radial muscles causes the
cytoelastic succulus to expand and display changes in color. Alternatively,
relaxation of the radial muscles leads to the retraction of the sacculus, thus
restoring the resting color pattern of the cuttlefish.

A visual to help you understand the chromatophore mechanism. The blue balloon acts as the cytoelastic succulus, and the strings mimic the radial muscles

In order to camouflage with its background, cuttlefish
must match with distinctive background characteristics such as the size
and spatial frequency of substrate objects, the degree of contrast, texture, color,
and pattern. That’s an immense task for any creature to take on, but the
cuttlefish is an expert. The distinct
body patterns in the cuttlefish due to changes in chromatophores have been partitioned
into 3 different patterns:

1.Uniform: Vary in color and
brightness, but attributes remain constant

2. Mottle: Characterized by small to
moderate light and dark patches (called mottles) that are evenly distributed
across the body surface. These patches generally match with corresponding
background objects.

3. Disruptive:
Large light
and dark components that exist in various shapes, scales, orientations, and
contrasts.

A: Uniform
B: Mottle
C: Disruptive
Can you spot the cuttlefish in the bottom three pictures?

These creatures are pretty awesome, right? Hang on though,
because things are about to get wild. Get this: cuttlefish are color blind. WHAT?!
You may wonder how these creatures are able to perform such drastically
different camouflage patterns across a wide array of visual backgrounds. It has
been proposed that additional cells in the skin of the cuttlefish aids in
camouflage. Aside from chromatophores, the skin of the cuttlefish also contains
cells that reflect specific wavelengths of light and ambient light, iridophores
and leucophores, respectively. Iridophores are found within a layer underneath
the chromatophores. They are stacks of platelets that are responsible for
giving cuttlefish and other organisms their metallic green, blue, and gold
appearance. Iridophores may change colors slowly in response to hormones.
Additionally, leucophores are maintained in a layer beneath both the
iridophores and chromatophores. These cells are responsible for white spots
that occur on cuttlefish. Leucophores may reflect either white or blue light
depending on which is the predominant ambient light.

Left: Iridophore as seen by the iridescent colorationRight: Leucophore are the white spots on the skin

In addition to these specialized cells, a study found the
opsin gene expressed in the skin of cuttlefish. Opsins, reflective and
light-sensitive proteins normally found in the retina of the eye, may also be
involved in camouflage. They proposed that these proteins may help the
cuttlefish detect reflectance properties of the environment. This would
influence chromatophore expansion and aid in matching the brightness of its
background. Additionally, opsins may act as filters and convey wavelength
information. Unfortunately, the exact function of the skin opsins remains to be
determined.

After all that effort the cuttlefish puts into camouflage, it
appears that its ability to evade predators lies in the eyes of the beholder
rather than the superpowers of the cuttlefish. Much of the information that
allows a predator to differentiate between the cuttlefish and background relies
more heavily on the brightness rather than the chromatic aspect of light. Well,
hopefully at least you’re impressed with the cuttlefish.

He's out to get you if you don't think he's awesome.

A final fascinating fact about the cuttlefish is the its
ability to produce pigmentation patterns called “passing clouds”. It is thought
that these cuttlefish possess this ability as a mean to hypnotize their prey.
In the passing cloud phenomenon cuttlefish are capable of propagating a
distinct dark band across different regions of their body. This band travels at
different frequencies as well as region-specific directions on the cuttlefish;
however, the bands are synchronized in identical regions. Not much research has been done on the
cuttlefish hypnosis attack, but hopefully it’ll be a topic of interest in the
near future.

Check out the cuttlefish trying to hypnotize prey.

All in all, would it be too much to say that cuttlefish are the kings of camouflage?
No? I didn’t think so either. They’re the color magicians of the deep abyss
that we call the ocean.

Finally, I leave you once again with Ze Frank and his true facts. On today's episode, "True Facts about the cuttlefish". Enjoy.

References:

Chiao, C., J.K. Wickiser,
J.J. Allen, B. Genter, R.T. Hanlon. 2011. Hyperspectral imaging of cuttlefish
camouflage indicates good color match in the eyes of fish predators.
Proceedings of the National Academy of Sciences of the United States of America
108:9148-9153.